Method and apparatus for determining current density



Jan. 19, 1965 H. HENTZ 3,166,484

METHOD AND APPARATUS FOR DETERMINING CURRENT DENSITY Original Filed April 17, 1958 POWER SUPPLY /Z5 VARmc INVENTOR. HOWARD J. HEN'rz ATTORNEY United States Patent Ofiice 3,155,484 Patented Jan. 19, 1965 Howard .ll. Hunts, South Bend, End, assignor to The Bendix Corporation, South Bend, ind, a corporation of Delaware Continuation of application tier. No. 72%,169, Apr. 1"],

1958. This application Apr. 2%, 11962, Ser. No. 189,207 Claims. (Ci. 204 l) This invention relates in general to electrolytic processing of work and in particular to the control of the current density of said work. This application is a continuation of copending application Serial No. 729,169, filed April 17, 1958, now abandoned.

My invention is particularly useful in the formation of aluminum oxide hard-coat deposited on aluminum alloy workpieces of irregular shape such, for example, as certain fuel control units used in an aeroplane engine. One of the well known electrolytic hard-coat oxidizing mechanisms, no claim to which is made per se, requires the workpiece of aluminum alloy to be treated in the electrolyte at a current density of say 36 amperes per square foot. Now this requires that the irregular shaped area to be coated be known; and that the voltage applied result in the proper current density for that area. The area to be hard-coated may be correctly estimated for workpieces of relatively simple shape; however it is not practical to attempt to figure the area of workpieces of irregular shape. The coating thickness attained is a function of ampere minutes per unit area; and an error in estimated area of the irregular shaped workpiece results in an error in applied coating thickness. Hardcoating of irregular shaped workpieces has been done by setting a voltage for a current density predetermined by experience, taking an ammeter reading and modifying this reading to approximate a predetermined voltage curve. However, the starting voltage is, in this case, too critical; and unsatisfactory plating results are attained.

Accordingly my invention is directed to electrical control means of an electrolytic mechanism, said control means serving to effect, by the operation of the mechanism and control, the desired current density of a load of unknown area being hard-coated; to the end that said load, that is work, is properly processed.

A further object of my invention is to provide, in an electrolytic mechanism for hard-coating aluminum alloy workpieces of unknown area, means, easily serviced and controlled, for controlling the density of current passing through the workpiece; and this control means of my invention supplements the well known electrical means of the electrolytic mechanism without modifying the same by change of construction.

Other objects of my invention and desirable details of construction of parts will become apparent from the following detailed description of an illustrative embodiment of the invention, taken in conjunction with the accompanying drawing illustrating said embodiment, in which the single figure of the drawing is a diagrammatic view of an electroplating mechanism which includes the control means constituting my invention.

This figure discloses a preferred embodiment of my invention in which a bath unit Ill includes a multisectioned cathode 11 of copper, a plurality of irregular shaped aluminum alloy workpieces 14 of unknown area constituting the anode, and an electrolyte 15 of 12% sulphuric acid by weight and 1% oxalic acid by weight. The acid amount per 100 gallons of electrolyte is 7.5 gallons of sulphuric at 66 F. and 11.75 lb. of oxalic acid crystals. During operation of the mechanism the oxalic acid will be used up and must be replaced. The oxalic acid content should not go below 0.5%.

As disclosed in the drawing the several workpieces 14, that is the anode of the unit 10, are wired to an electrical source of supply 16 preferably a generator; and the several sections of the cathode are also wired to said generator. An ammeter 18, in series in the circuit comprising the .generator, anode, cathode and electrolyte, serves to indicate the amperage of the electrical current flowing into and from the bath unit 10; and a voltmeter 2d serves to indicate the voltage applied to the workpieces 14 and to a control means generally indicated by the numeral 22. This control means constituting the most important feature of my invention, comprises an aluminum alloy anode member- 24 of known area, say 2 square inches. This member, which is of the same alloy as the workpieces '14, is immersed in the electrolyte 15 and lies in an electrical circuit which is in parallel with the circuit including the workpieces 14; and an ainrneter 2 6 in this parallel circuit serves to indicate the amperage of the current passing through the known area anode member 24.

In general, the resistance of aluminum oxide coatings to abrasion and wear is dependent on two factors; namely, the thickness of the oxide coating and the conditions under which the coating is formed. While the abrasion and wear resistance of an aluminum oxide coating is approximately proportional to the thickness thereof, the structure of said coating itself is a most important factor in the performance of the coating when subjected to abrasion or wear. This structure is influenced chiefly by the operating conditions employed to form the coating, such as the type of electrolyte, temperature of operation, uniformity of temperature and the rate of oxide formation.

Thus, any conditions during electrolytic treatment which cause appreciable dissolution of the oxide coating, such as for example, immersion in the electrolyte at an elevated temperature, or for very long coating periods, reduce the abrasion and wear resistance of the coating. On the other hand, denser and more abrasion and wear resistant coatings are obtained by employing an electrolyte having low solvent action on the coating and by using a low operating temperature to further decrease dissolution of the coating during treatment. To obtain consistent results, it is necessary to provide adequate means for agitating the electrolyte in order to keep the temperature of the electrolyte uniform and to prevent local overheating.

Having in mind the aforementioned conditions under which the hard-coat coating is to be formed my invention lies in the control of the current density of the load being coated. In my invention, that is the combination of elements disclosed in the drawing, the area of the load is ignored and the amperage on the known area member 24 in the parallel circuit is controlled. This amperage is controlled by controlling the voltage applied to the known area member 24; and it is to be noted that this same voltage is also applied to the workpieces. This voltage control is preferably eifected by the operation of a variac 28 which controls the generator output. Omitting the ammeter 26 the resistance of the wiring of each of the parallel circuits is the same or substantially the same.

Assuming that a current density of 36 amperes per square foot is desired and that the known area 24 is made 2 square inches then the variac 28 is adjusted to effect a 0.5 amperage reading of the ammeter 26. Assuming a total amperage of 90.5 as read by the ammeter 18 it follows that we have amperes flowing through the unknown area, that is workpieces l4; and the current density of the known and unknown areas is then 0.25 per square inch that is 36 amperes per square foot.

Briefly describing the operation of the mechanism of my invention the work, a ter having been first cleaned, is, together with the known area member 24, placed in ares res the electrolyte with the generator either off or at a minimum setting. Current is then applied gradually to polarize the work and to establish the required current density. As indicated hereinafter during the cycle frequent small increases in the voltage will be required to maintain a uniform current density. Maintenance of the proper temperature of the electrolyte is an important factor, furthermore as stated above rapid agitation of the electrolyte is required to maintain the proper temperature to avoid local heating. Rinsing after the anodic treatment constitutes the final step in the production of the hard coating on the work.

There is thus provided, by the mechanism of my invention, electrolytic means for effecting the desired aluminum oxide hardcoating of one or more workpieces of aluminum alloy, said workpieces being of unknown area and preferably constituting the anode of the said electrolytic means. With my invention the current density of the workpieces, and therefore the desired hardcoat of said workpiece, is determined by controlling the amperage of the known area 24. To prevent burning on some alloys and certain types of articles the current density may be reduced from a factor of 36 to about 24 aniperes per square foot and the time of treatment extended to obtain the total number of ampere-minutes.

It is also to be noted that the generator output, measured by the voltage of the voltmeter 29, must be orogressively increased with time; for the coating on the workpieces is a dielectric and requires more voltage to maintain a constant amperage as the coating gets thicker.

ther factors which must be taken into account and allowed or compensated for are (l) the changing resistivity of the electrolyte and (2) the fact that the ammeter 26 in the parallel, that is, control circuit, is a resi tance in series with the known area part 24.

My invention disclosed herein lies in the combination of the above described parallel or so-called control circuit with the remainder of the mechanism disclosed: I make no claim to this latter mechanism per se.

While the preferred embodiment of the invention has been described in considerable detail, I do not wish to be limited to the particular construction shown which may be varied within the scope of the invention, and it is my intention to cover hereby all adaptations, modifications and arrangements thereof which come within the practice of those skilled in the art to which the invention relates.

I claim:

1. In electrolytic mechanism having an electrolytic tank adapted to house an electrolyte, the combination of a source of electricity,

a first electrode housed within the tank and connected to said source,

a second electrode including a workpiece of unknown area positioned within the and connected to said source,

a third electrode of known area and of the same material as said second electrode positioned within the tank and connected to said source in parallel with said second electrode,

and means opcratively connected to said third electrode for determining the electrical current flow therethrough whereby the current density of said known area is known,

said known current density being the same as that of said workpiece of unknown area.

2. In electrolytic mechanism having an electrolytic tank adapted to house an electrolyte, the combination of a source of electricity, a cathode positioned within the tank and connected to said source,

a first anode including a workpiece of unknown area positioned within the tank and connected to said source,

a second anode of known area and of the same material as said workpiece of unknown area arranged in parallel with said first anode,

and means operatively connected to said second anode for determining the electrical current flow therethrough whereby the current density of sai known area is known,

said known current density being indicative of that of said unknown area.

3. In electrolytic mechanism having an electrolytic tank adapted to house an electrolyte, the combination of an anode housed within the tank,

said anode comprising a workpiece of irregular shape and unknown area,

a cathode positioned within the confines of the tank,

an additional anode positioned within the tank,

said anode being of known area and formed of the same material as said first named anode,

a cor rollable source of electrical power,

an electric circuit having an ammeter inserted therein and in which the workpiece is inserted,

an independent electric circuit in parallel with that portion of the aforementioned circuit which includes the workpiece, said independent circuit having the known area anode inserted therein and also an ammeter inserted therein such that the current flow through said independent current thus the current density of said known area anode and said electric circuit which contains said unknown area workpiece is determinable,

and means for controlling said source of electrical power to produce a current flow corresponding to a desired current density.

4. The method of determining the electrical current density of a workpiece of unknown area in an electrolyte comprising the steps of:

immersing an electrode of known area and of the same material as the workpiece of unknown area in the electrolyte, connecting said electrode of known area in parallel electrically with the workpiece of unknown area,

inserting an ammeter in said parallel connection to thereby measure the electrical current flow through said electrode of known area,

determining the electrical current density of said electrode of known area whereupon the electrical current density of the workpiece of unknown area is known, and

controlling said electrical current flow to a predetermined value to thereby establish a corresponding predetermined electrical current density.

5. The method of determining the electrical current density or" a workpiece of unknown area in an electrolyie comprising the steps of:

immersing an electrode of known area and of the same material as the workpiece in the electrolyte, connecting said electrode of known area in parallel electrically with said workpiece,

inserting an amrneter in said parallel connection to thereby measure the electrical current flow through said electrode of known area,

determining the electrical current density of said electrode of known area whereupon the electrical current density of the workpiece is known, and controlling said electrical current flow to a predetermined value to thereby establish a corresponding predetermined electrical current density.

6. The method of determining the electrical current density of a workpiece of unknown area in an electrolyte comprising the steps of immersing the workpiece of unknown area in the electrolyte,

immersing an electrode in the electrolyte,

connecting the workpiece and said electrode to a source of electrical power to establish opposite polarity between the workpiece and said electrode,

immersing an electrode of known area and of the same material as said workpiece of unknown area in the 03) electrolyte and connecting the same source of electn'cal power in parallel with said workpiece and in series with said first named electrode,

inserting an ammeter in said parallel connection for measuring the electrical current flow through said electrode of known area, determining the current density of said electrode of known area whereupon the current density of said workpiece of unknown area is known, and

controlling said source of electrical power to establish a predetermined electrical current fiow through said electrode of known area and thus said workpiece of unknown area whereupon a corresponding predetermined current density through said electrode and workpiece is established.

7. The method of determining the electrical current density of a workpiece of unknown area in an electrolyte comprising the steps of:

immersing the workpiece of unknown area in the electrolyte,

immersing an electrode in the electrolyte,

connecting the workpiece and said electrode to a source of electrical power to establish opposite polarity between the workpiece and the electrode, immersing an electrode of known area and of the same material as the workpiece in the electrolyte in parallel electrically with the workpiece and in series electrically with the first named electrode,

inserting an amrneter in said parallel connection for measuring the electrical current flow through said electrode of known area,

determining the electrical current density of said electrode of known area whereupon the electrical current density of said workpiece is known, and

controlling said electrical power to increase or decrease the elcctrical current flow through said electrode of known area as reqiured to increase or decrease said cunrent density to a desired value.

8. The method of determining the electrical current density of a workpiece of unknown area in an electrolyte comprising the steps of:

immersing the workpiece of unknown area in the electrolyte and connecting the same to a source of electrical power to establish an anode,

connecting said source of electrical power to said electrolyte by means including a cathode,

immersing an anode of known area and of the same material as said workpiece of unknown area in the electrolyte and connecting the same in parallel electrically with the workpiece,

inserting an ammeter in said parallel connection for measuring the electrical current flow through said anode of known area,

determining the electrical current density of said anode of known area whereupon the electrical current density of the workpiece is known, and controlling said electrical power to increase or decerase the measured electrical current flow and thus increase or decrease said current density to a desired value. L 9. In electrolytic mechanism having an electrolytic tank adapted to house an electnolyte, the combination of:

a source of electricity; an anode an a cathode positioned within the tank and operatively connected to said source of electricity; one of said anode and cathode constituting a workpiece of unknown area; I an electrode of known area and of the same material as said workpiece of unknown area connected to said source in parallel with said workpiece of unknown area and means operatively connected to said electrode for de termining the electrical current flow therethrough whereby the current density of said known area is known; said known current density being the same as that of said workpiece of unknown area. 10. In electrolytic mechanism having an electrolytic tank adapted to house an electrolyte, the combination of:

a controllable source of electrical power; an anode within the tank comprising a workpiece of irregular shape and unknown area operatively connected to said controllable source of electrical power; a cathode within the tank and operatively connected to said controllable source of electrical power; an additional anode within the tank and operatively connected to said controllable source of electrical power in parallel with said first named anode, said additional anode having a known area and formed of the same material as said first named anode; means operatively connected to said additional anode for measuring the electrical current flow therethrough; and means for controlling said source of electrical power to maintain said electrical current flow substantially constant whereby the current density of said additional anode and thus said first named anode in parallel therewith is maintained at a corresponding substantially constant value.

References Cited in the file of this patent UNITED STATES PATENTS 542,057 Hulin July 2,1895

OTHER REFERENCES Electroplating Engineering Handbook, 1955, page 618. 

1. IIN ELECTROLYTIC MECHANISM HAVING AN ELECTROLYTIC TANK ADAPTED TO HOUSE AN ELECTROLYTE, THE COMBINATION OF A SOURCE OF ELECTRICITY, A FIRST ELECTRODE HOUSED WITHIN THE TANK AND CONNECTED TO SAID SOURCE, A SECOND ELECTRODE INCLUDING A WORKPIECE OF UNKNOWN AREA POSITIONED WITHIN THE TANK AND CONNECTED TO SAID SOURCE, A THIRD ELECTRODE OF KNOWN AREA AND OF THE SAME MATERIAL AS SAID SECOND ELECTRODE POSITIONED WITHIN THE TANK AND CONNECTED TO SAID SOURCE IN PARALLEL WITH SAID SECOND ELECTRODE, AND MEANS OPERATIVELY CONNECTED TO SAID THIRD ELECTRODE FOR DETERMINING THE ELECTRICAL CURRENT FLOW THERETHROUGH WHEREBY THE CURRENT DENSITY OF SAID KNOWN AREA IS KNOWN, SAID KNOWN CURRENT DENSITY BEING THE SAME AS THAT OF SAID WORKPIECE OF UNKNOWN AREA. 